Rails 3: How can I improve this ActiveRecord count

Question

Here I retrieve a collection of causes from the database. Along with the information from each cause, I want to send the number of users in each cause. This way works, but if experience has told me anything there is probably a much more efficient method to do this. Any advice would be greatly appreciated.

  def index
    @causes = Cause.near([params[:lat], params[:lng]],10)
    @causes.each do |i|
      i['count'] = i.users.count
    end
    respond_to do |format|
      format.json { render json: {:success=> 1, :data_type => 'cause',:results => @causes} }
    end
  end

Thanks!

Update:

I tried the solution below but it is not working.

  Cause.select('causes.id,count(users) as
  people').joins(:users).group('causes.id')

I can't get any of my aliases(AS) to show up. Results:

Cause Load (0.8ms)  SELECT causes.id,count(users) as people FROM "causes" INNER JOIN "cause_user_memberships" ON "cause_user_memberships"."cause_id" = "causes"."id" INNER JOIN "users" ON "users"."id" = "cause_user_memberships"."user_id" GROUP BY causes.id

 => [#<Cause id: 25>, #<Cause id: 27>, #<Cause id: 12>, #<Cause id: 19>, #<Cause id: 1>, #<Cause id: 2>, #<Cause id: 3>, #<Cause id: 23>, #<Cause id: 26>, #<Cause id: 11>, #<Cause id: 4>, #<Cause id: 20>, #<Cause id: 13>, #<Cause id: 24>] 

Any advice? I'm using postgres. Thanks again!

Update 2:

User.rb (snippet)

has_many :cause_user_memberships
has_many :causes, :through => :cause_user_memberships

Cause.rb

attr_accessible :title, :location, :description,...
has_many :cause_user_memberships
has_many :users, :through => :cause_user_memberships

Cause_User_Membership.rb (<--probably not my best model name)

    # == Schema Information
#
# Table name: cause_user_memberships
#
#  id         :integer          not null, primary key
#  user_id    :integer          not null
#  cause_id   :integer          not null
#  created_at :datetime         not null
#  updated_at :datetime         not null
#

class CauseUserMembership < ActiveRecord::Base
  attr_accessible :cause_id, :user_id

  belongs_to :user #, :class_name => "User", :foreign_key => "user_id" 
  belongs_to :cause #, :class_name => "Cause", :foreign_key => "cause_id"

end

Last Update: Success

EXPLAIN (1.3ms)  EXPLAIN SELECT causes.id,count(users) as people FROM "causes" INNER JOIN "cause_user_memberships" ON "cause_user_memberships"."cause_id" = "causes"."id" INNER JOIN "users" ON "users"."id" = "cause_user_memberships"."user_id" GROUP BY causes.id
EXPLAIN for: SELECT causes.id,count(users) as people FROM "causes" INNER JOIN "cause_user_memberships" ON "cause_user_memberships"."cause_id" = "causes"."id" INNER JOIN "users" ON "users"."id" = "cause_user_memberships"."user_id" GROUP BY causes.id
                                         QUERY PLAN
--------------------------------------------------------------------------------------------
 HashAggregate  (cost=51.26..51.36 rows=10 width=4212)
   ->  Hash Join  (cost=20.90..51.21 rows=10 width=4212)
         Hash Cond: (cause_user_memberships.cause_id = causes.id)
         ->  Hash Join  (cost=10.22..40.17 rows=70 width=4212)
               Hash Cond: (cause_user_memberships.user_id = users.id)
               ->  Seq Scan on cause_user_memberships  (cost=0.00..24.00 rows=1400 width=8)
               ->  Hash  (cost=10.10..10.10 rows=10 width=4212)
                     ->  Seq Scan on users  (cost=0.00..10.10 rows=10 width=4212)
         ->  Hash  (cost=10.30..10.30 rows=30 width=4)
               ->  Seq Scan on causes  (cost=0.00..10.30 rows=30 width=4)
(10 rows)

 => [{"id"=>25, "people"=>"1"}, {"id"=>27, "people"=>"2"}, ...]

Answer 1

You can perform an inner-join on the users table and then group by the causes.id.

@causes = Cause
  .near([params[:lat], params[:lng]], 10)
  .joins('users')
  .group('causes.id')
  .select('causes.*, count(users) as count')